I think the takeaway here is that neither of the perspectives we've seen, Sei's throughout the series or his mother's in the last two chapters, can be taken as objective reality. Whatever the "reality" of this sequence is (and I'd be surprised if we get a clear answer on this point), Sei is seeing things from his mother's perspective.
Just a few chapters ago Sei finally vented his repressed feelings of anger and resentment toward his mother. Now we see that, to his mother, those feelings have always been obvious.
I think the facts of this story are clear- Seiko pushed Shige off the cliff and lied about it. After this, she alienates her family, apart from Sei, to whom she is emotionally abusive and manipulative. But our perspective character has a distorted view of her mother, and has led us to certain assumptions about her state of mind, her motives, or her attitude.
It's irresponsible to diagnose Seiko, but some of her symptoms point to post-partum depression, DID, schizophrenia, or any number of disorders. The last few chapters show a woman who is trapped in her life, smothered by feelings of guilt, and terrified of her son. These things have always lurked under the surface of this story, but now they're made explicit.